Production of uranium



2,313, lb Patented Nov. 12, 1957 PRODUCTION OF URANIUIVI America as represented by the United States Atomic I Energy Commission No Drawing. Application July 14, 1955, Serial No. 522,190

(Ilaims. (Cl. 7584.1)

This invention deals with an improved process of producing uranium metal by the reduction of uranium tetrafluoride with magnesium in an autoclave.

Heretofore for this purpose a mixture of uranium tetrafluoride and magnesium was heated in a steel autoclase or bomb which had been lined with ground dolomite to protect the steel from corrosion. However, a number of disadvantages were encountered. The magnesium fluoride slag which was formed in the process reacted with the liner, and magnesium oxide, calcium fluoride and/ or calcium oxide were thus formed; these side products were taken up by the slag and the fluidity of the latter was reduced and its purity impaired thereby. The presence of the side products in the slag complicated the uranium recovery therefrom by acid leaching.

Another disadvantage was that the dolomite, due to the water content present caused hydrolysis of the uranium tetrafluoride and conversion to uranium dioxide which also was taken up by the slag. Since uranium dioxide is not as easily reduced with magnesium as the uranium tetrafluoride, a low yield and thus a loss of uranium were the result.

Another undesirable reaction which was frequently encountered in the bomb process for the production of uranium metal was the formation of magnesium vapor at temperatures lower than the reaction temperature proper and the ensuing reduction of the uranium tetrafluoride by the magnesium vapor to the uranium trifluoride. Uranium trifluoride, like uranium trioxide, is reduced by magnesium metal with difliculty only so that the formation of magnesium vapor also impaired the total yield of the process. The formation of uranium trifluoride quasi was a premature start of the reaction whereby part of the reaction heat was developed at an early stage rather than at the time of reaction proper and ignition. This spread of the reaction heat over a greater time interval also made the process less eflicient.

It has been tried to replace the dolomite by other materials in order to eliminate the disadvantages just described. The substitution of the magnesium fluoride slag formed in the process for the dolomite liner, for instance, has been investigated. However, for some unknown reason the use of slag as the liner of the bomb always resulted in a reduction of the yield, this so, in particular, when the reaction and/or firing temperatures were relatively high. (The temperature at which the heating curve, time plotted against temperature, first showed an increase in slope was considered the reaction temperature. The temperature at which simultaneous measurements of temperature and time became impossible because the change was too rapid was considered the ignition or firing temperature.) A high firing temperature is desirable because then the slag is more fluid and its separation from the metal formed is more complete than it would be at lower temperatures.

Apart from a good phase separation and a high firing temperature, there are other factors which are of importance in the production of uranium by the bomb process. For instance, the period of time required to heat the charge to firing temperature should be as short as possible so that there is as little time available as possible for dissipation of the reaction heat. Of course, the properties of the metal obtained determine the efficiency of the process; for instance, the purity and physicalcondition of the metal have to be considered and also, as has been implied above, the yield.

It is an object of this invention to provide a process for the production of uranium metal by reacting uranium tetrafluoride with magnesium whereby a high yield is obtained.

It is another object of this invention to provide a process for the production of uranium metal by reacting uranium tetrafluoride with magnesium whereby a high yield is obtained even when the reaction bomb is lined with the slag produced in the process.

It is still another object of this invention to provide a process for the production of uranium metal by reacting uranium tetrafluoride with magnesium that is economical.

Another object of this invention is to provide a process for the production of uranium metal by reacting uranium tetrafluoride with magnesium in which good utilization of the reaction heat is accomplished, a fluid slag and good separation of the metal produced are obtained.

Another object of the invention is to provide a process for the production of uranium metal by reacting uranium tetrafluoride with magnesium in which the firing temperature is relatively high and the firing time short.

'Still another object of this invention is to provide a process for the production of uranium metal by reacting uranium tetrafluoride with magnesium in which no side products are formed which would complicate the uranium recovery by leaching the slag formed with acid.

It is finally also an object of this invention to provide a process for the production of uranium metal by reacting uranium tetrafluoride with magnesium in which substantially no magnesium vapor at temperatures lower than reaction temperatures and thus no uranium trifluoride are formed.

These objects, according to this invention, are accomplished by providing the magnesium with a surface fihn so that undesirable and premature reactions cannot take place or are stopped in their very initial stage. The film is formed by heating the magnesium with substances which decompose or react with the magnesium at temperatures below the reaction temperature to form a film on its sur face which is stable below reaction temperature. Such film-forming additives found suitable are uranium trioxide, hydrated uranium tetrafluoride, uranyl fluoride, and sodium bifluoride; most likely the films consist of magnesium oxide, magnesium fluoride, or magnesium oxyfluoride.

The use of uranium trioxide as the additive was found particularly advantageous whenever a metal of high purity was to be obtained since the uranium trioxide is available in very pure form. The uranium trioxide at elevated temperature probably disproportionates to form U308 and oxygen, and the oxygen then reacts with the magnesium and forms the oxide film on its surface. Uranium trioxide was found to be the best additive of all the compounds tested.

Sodium bifluoride when added as the film-forming compound has a dual function. It decomposes at elevated temperature into hydrogen fluoride and sodium fluoride. The sodium fluoride lowers the melting point and the viscosity of the slag and thereby improves phase separation between the metal and the slag and thus metal recovery. The hydrogen fluoride provides a film of magnesium fluoride on the surface of the magnesium.

The quantity of the film-forming additive ranges preferably between 0.5 and 2% by weight of the uranium tetrafluoride; however, quantities outside this range have alsobeen found operative.

In the conventional bomb process, without the use of an additive and at a heating rate of about 20 to 40 C. per minute, the reaction temperature is between about 550. and 600 C. When a reaction temperature of above 600 C. is observed, the formation of a film is indicated.

The quantity of magnesium, of course, has to be at least the amount stoichiometrically required; but a slight excess, preferably of between 3 and 5% of the stoichiometric amount, is desirable for best operation.

In order to produce a film on the magnesium, the latter can either be pretreated separately by mixing and heating it with the additive, or else the film-forming agent may be admixed to the charge for the bomb; inthe latter case filming takes place in situ. Both methods have given good results. a a

Instead of simply mixing the additive with the uranium charge, it was found especially advantageous to incorporate the additive in the form of compressed wafers. For this purpose the additive is mixed with magnesium (4% excess) and then small portions of the mixture are compressed inside a die of a hydraulic press. For instance, wafers which had been formed from 25 gms. of the additive-magnesium mixture by applying a pressure of about 10,000 p. s. i. have given very good results. It was found that by using these wafers instead of a plain mixture the firing time was reduced without impairing the yield.

The structure of the bomb is not critical, and any autoclave known to those skilled in the art is operative. However, a cylindrical steel shell was preferably used which had a curved, dish-shaped, bottom and a flanged top. Below the bottom a skirt was arranged to allow the bomb to stand upright. A lid was bolted to the top flange during reaction. No gaskets were used.

The bomb was lined with powdered dolomite or magnesium fluoride slag derived from the process. 'The latter was preferred. It was preferably ground fine enough that at least about 75% passed through a IOO-mesh screen. Lining of the lower part of the bomb (bottom and lower part of the sides) was carried out by using a bottom liner form of a smaller width than that of the bomb and tamping the liner material into the interstice left between the shell and the bottom liner form. Tamping was carried out by means of an air-driven jolter. For the upper part a mandrel was used; the sides of the liner thus formed were also packed by tamping. Into the bomb thus lined the blended charge was added and also tamped. Finally a top liner, to fill the space between the top of the charge and the top of the bomb, was added and hand tamped with a mallet and a circular wooden tamper. Then the bomb was closed with the lid and inserted into a furnace to bring it up to reaction temperature. 7

As has been mentioned before, a slag liner is preferred to one formed of dolomite; however, one drawback of slag is that it does not pack as well as dolomite with the mandrels of steel used heretofore. The slag liner produced thereby was found to be too soft and not to adhere to the sides sufiiciently. The consequence of this was that the cavity for the charge did not retain its even shape and the metal piece obtained after the reaction, the derby, which separated at the bottom of the bomb, had irregular dimensions and rough surfaces.

It was found that by using a mandrel of resilient material this could be overcome. A springy rubber that was so hard that it appeared rigid to the thumb of a person testing it was used as the mandrel material. The diameter of the rubber mandrel was slightly less (about 1 inch less) than the inner width of the bomb and was essentially cylindrical in shape. Its bottom was closed with the exception of a hole into which a stopper rod could be introduced from the top of the mandrel. 1

When such a rubber mandrel was used, the bottom lining of the bomb was first applied, as has been described above, using a steel bottom liner form. Thereafter the rubber mandrel was inserted concentrically and the-annular space between the mandrel and the bomb was filled;

with ground slag. The assembly was then jolted for about 20 minutes and more slag was added as necessary to fill the interstice. Thereafter the mandrel was removed; for this purpose the mandrel stopper rod was first lifted. By this the bottom of the mandrel was opened and air had access to the space between the bottom of the bomb and the bottom of the mandrel as the liner was withdrawn. This arrangement prevents the formation of a vacuum during the withdrawal of the mandrel and thus. the collapse of the side lining.

When the rubber mandrel is jolted in the vertical direction it expands laterally due to this vertical pressure and thereby exerts a horizontal force on the lining powder. Consequently a greater hardness is obtained than when a conventional steel mandrel is used which merely exerts a compression in the vertical direction. A rubber mandrel, due to its higher eificiency, requires less weight than a steel mandrel. For instance, an '85-lb. rubber mandrel yielded a liner that was packed better than one made with a 243-lb. steel mandrel.

Another advantage of the rubber mandrel is that it can be withdrawn without spoiling the lining. This is possible because the rubber, after the jolting force has ceased, springs back into its original form and thereby becomes detached from the lining. Thus, it can be withdrawn without hammering which is requiredin the case of steel mandrels.

After the bomb had been lined and charged as described above, it was evacuated for about 3 /2 hours with a two-stage steam ejector; it was then sealed and fired. The vacuum obtained averaged about 28.5 inches of mercury. Of course, when the bomb was then heated, the pressure increased. When the bomb was removed from the furnace after firing, the average pressure was 30 p. s. i. g. When the cold bomb was opened, the slag was at the top and a massive metal piece, the derby, at the bottom of the bomb. The derby had some slag attached to its surface but this could be easily removed by simply chipping it oif. V

Frequently some small particlesof the metal formed did not enter the derby but were trapped in the slag. These pellets, which were separated from the slag, after disintegration, by screening or the like, were usually covered with magnesium and magnesium fluoride which made remelting in a vacuum difiicult. It was desirable, of course, to recover the uranium of the pellets by re cycling them into the bomb. This couldbe satisfactorily effected after the pellets had been subjected to a pickling treatment, e. g., with nitric acid. The pellets were preferably made the bottom layer of the bomb charge.

When uranium trioxide was used as the additive, the slag usually contained some magnesium oxide and the Oxide content steadily increased with each reuse or gen eration of the slag because each time a new portion of uranium trioxide was added the magnesiumoxide content increased, the melting point of the slag was thus raised until it was too high for satisfactory slag separation (the melting point of magnesium oxide is 2800 C., that of magnesium fluoride is .1396 C.).

In order to avoid uranium losses caused by the reaction just described, the slag was reconditioned by adding sodium fluoride to the bomb charge from time to time.

A quantity of about 2.5% by weight of sodium fluoride 6 minutes, while for the control run the yield was 96% i and the firing time almost 4 hours. i

In the following a number of examples are given to illustrate th process Of'the invention without the ins tention to limit the scope of the invention to the details given therein.

Example I Several charges of finely divided magnesium were heated in a container, each together with a filming agent, to a temperature of roughly 550 to 600 C. The charge was surrounded by a stationary helium atmosphere. After 2 hours the container was cooled while the helium atmosphere was maintained. The run with sodium bifluoride as the addition was carried out in the presence of air, without using helium. The magnesium thus pretreated was then mixed with uranium tetrafluoride and charged into a bomb and reacted there as described above. The reaction as well as the ignition temperatures were measured. The results are compiled in the table below.

This table shows that the reaction temperature was considerably increased by the treatment of the magnesium with the additives of this invention.

Example II In this instance uranium trioxide was used as the filmforming additive; it was admixed to the charge of the bomb, and the film was thus formedin situ. The bomb was lined with slag and the slag was used through ten generations; this means that the slag formed in each of the runs was mixed with the liner slag and part of the mixture was used as the liner material. The charge consisted of 170 lbs. of uranium tetrafluoride, 27 lbs. of magnesium, and about 1.5 lbs. of uranium trioxide. The furnace temperature was held at about 665 C. In the first three runs the yield was approximately 98%, but it then dropped until it was 96.3% in the tenth generation. The purity of the metal, however, was about the same in all ten runs.

As set forth above, the reason for this reduction in yield is the increased melting point of the slag which can be easily controlled, however, by the addition of sodium fluoride to the charge.

Example III It will be understood that this invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

in an autoclave to a temperature of about 665 to 735 C.'

whereby metallic uranium forms and separates from a slag phase, said additive being present in an amount of from 0.5 to 2% of the quantity of uranium tetrafluoride.

2. The process of claim 1 wherein the charged autoclave is evacuated prior to heating.

3. The process of claim 1 wherein the magnesium is treated with the film-forming additive separately and 7 prior to mixing it with the uranium tetrafluoride.

4. The process of claim 2 wherein the quantity of the magnesium is excessive by 3 to 5% over the amount stoichiometrically required.

5. The process of claim 4 wherein the additive is uranium trioxide.

6. The process of claim 4 wherein the additive is sodium bifiuoride.

7. The process of claim 1 wherein film-forming is carried out in situ by admixing the additive to the bomb charge.

8. The process of claim 7 wherein the additive is incorporated into the charge in the form of wafers which have been compressed from a mixture of the additive and magnesium.

9. A process of producing metallic uranium by the reduction of uranium tetrafluoride with magnesium comprising heating the magnesium with an additive selected from the group consisting of sodium bifluoride, uranyl fluoride, hydrated uranium tetrafluoride, and uranium trioxide to a temperature of between 550 and 625 C. whereby a surface film is formed on the magnesium, and heating at least the stoichiometrically required amount of magnesium thus treated with the uranium tetrafluoride in an autoclave to a temperature of about 665 to 735 C. whereby metallic uranium forms and separates from a slag phase, said additive being present in an amount of from 0.5 to 2% of the quantity of uranium tetrafiuoride and the surfaces of the autoclave contacting the magnesium-uranium tetrafluoride charge being lined with magnesium fluoride slag obtained in a previous run of the process.

10. The process of claim 9 in which the slag is disintegrated prior to using it for lining the autoclave so that of it will have a maximum particle size of mesh.

No references cited. 

1. A PROCESS OF PRODUCING METALLIC URANIUM BY THE REDUCTION OF URANIUM TETRAFLUORIDE WITH MAGNESIUM COMPRISING HEATING THE MAGNESIUM WITH AN ADDITIVE SELECTED FROM THE GROUP CONSISTING OF SODIUM BIFLUORIDE, URANYL FLUORIDE, HYDRATED URANIUM TETRAFLUORIDE, AND URANIUM TRIOXIDE TO A TEMPERATURE OF BETWEEN 550 AND 625*C. WHEREBY A SURFACE FILM IS FORMED ON THE MAGNESIUM, AND HEATING AT LEAST THE STOICHIOMETRICALLY REQUIRED AMOUNT OF MAGNESIUM THUS TREATED WITH THE URANIUM TETRAFLORIDE IN AN AUTOCLAVE TO A TEMPERATURE OF ABOUT 665 TO 735*C. WHEREBY METALLIC URANIUM FORMS AND SEPARATES FROM A SLAG PHASE, SAID ADDITIVE BEING PRESENT IN AN AMOUNT OF FROM 0.5 TO 2% OF THE QUANTITY OF URANIUM TETRAFLUORIDE. 